There is an ongoing need to reduce the size of the elements within integrated circuits and semiconductor structures. As the size of an element is reduced, shorter wavelength radiation is needed to for exposing the photoresist in order to obtain the smaller features desired. Consequently, photoresists sensitive to the shorter wavelength radiation must also be used. In order to obtain features on the order of 0.1 micron, radiation having a wavelength of 193 nm is used, and the photoresists sensitive to this wavelength are referred to as 193 nm resists. A variety of these resists are commercially available, such as T9269 (from Clariant International, Ltd., Muttenz, Switzerland), 6A100 (or TOK 6100) (Tokyo Ohka Kogyo, Kawasaki-shi, Japan), and AR414 and AR237 (both from Japan Synthetic Rubber Co., Ltd., Tokyo, Japan).
After exposure of the resist, it is baked. This is referred to as post exposure bake. Typically, the post exposure bake involves heating the resist to 130° C. for 90 seconds. A disadvantage, however, is that the resist suffers from post exposure bake sensitivity, resulting in poor CD (critical dimension) control. Post exposure bake sensitivity is greater in 193 nm resists than in 248 nm resists.
FIG. 1A illustrates the CD control of this process. Here, a polysilicon layer was patterned on a set of wafers, to form semi-dense lines (the lines are 240 nm apart). Sixteen wafers were prepared. Each wafer was measured at 8 fields, and 6 data points were measured in each field of wafers 1, 8 and 16.
The vertical axis represents the width of these lines, while the horizontal axis represents the temperature of the post exposure bake. The shape of the data point correlates with the exposure energy. As can be seen, the CD varies with energy of exposure, and the slope of the line represents the post exposure bake sensitivity which is 6.71 nm/degree. FIG. 1B includes a point for each measurement taken, and more clearly illustrates the variation in CD. The 6 sigma value of wafer to wafer variation (the standard deviation for the average of each wafer, multiplied by 6) was 5.7 nm.